1. Field of the Invention
The present invention relates to a heat transfer system for and method of controlling a patient""s temperature.
2. Description of the Related Art
Man is an animal with a normal functioning body temperature of about 37xc2x0 C. Therefore, comfortable human survival requires a body""s core temperature of about 37xc2x0 C., +/xe2x88x92 perhaps 1xc2x0. See, xe2x80x9cHypothermiaxe2x80x94physiology, Signs, Symptoms and Treatment Considerationsxe2x80x9d, Search and Rescue Society of British Columbia, compiled by Michael McEwan, 1995. The McEwan article further notes that a body can self compensate for small upward or downward variations in temperature through the actuation of a built-in thermal regulatory system, controlled by temperature sensors in the skin.
For example, the response to an upward variation in body temperature is the initiation of perspiration, which moves moisture from body tissues to the body surface, where evaporation causes cooling. Likewise, the response to a downward variation in body temperature is shivering, which is the body""s involuntary contraction and expansion of muscle tissue on a large scale in an attempt to generate heat.
Stiff and Sixta, xe2x80x9cHypothermia Care and Preventionxe2x80x9d, 1997, generally define hypothermia as occurring when the body""s core temperature drops below its normal 37xc2x0 C. In contrast, the McEwan article defines impending hypothermia as occurring when the core temperature decreases to 36xc2x0 C.
In the early stages mild hypothermia causes vigorous shivering which is usually accompanied by an increase in pulse and breathing rates. Cold, white hands and feet (as the blood vessels in the skin constrict) are the first signs of blood being shunted away from the body""s extremities.
The McEwan article describes mild hypothermia as occurring when the core temperature is dropped to the range of 34-35xc2x0 C. At this point, uncontrolled, intense shivering begins, although the victim is still alert and able to help itself, however, movements become less coordinated and the coldness creates some pain. Hypothermia occurs when the core temperature is in the range of 31 to 33xc2x0 C. At this point shivering slows or stops, muscles begin to stiffen and mental confusion and apathy sets in. Speech becomes slow, vague and slurred with breathing becoming slower and shallower.
The McEwan article defines severe hypothermia as occurring when the body""s core temperature is below 31xc2x0 C.; Stiff and Sixta define severe hypothermia as resulting when the body temperature drops below 33xc2x0 C. Shunting of the blood continues, manifesting as bluish lips and finger tips from poor oxygenation of the tissues near the body surface. Decreased circulation results due to a build-up of acid metabolites (waste products) in the muscles of the extremities until shivering stops and is replaced by muscular rigidity. The pulse and respirations slow as the body core cools to about 30xc2x0 C. The heart may stop at temperatures of about 28xc2x0 C. or less.
Hypothermia can occur during any outdoor excursion, especially in wilderness situations where weather conditions may deteriorate unexpectedly or where travelers become lost, get injured or exhaust food supplies prematurely. Additionally, outdoor activities involving water present the added possibility of emersion with the body cooling up to 25 times faster in water than in air.
Mild hypothermia is also a common occurrence during major surgery on the body. The usual causes of such perioperative hypothermia or anesthetic-induced impairment of thermal regulation include exposure to cold, altered distribution of body heat, and surgical exposure of the body cavity to a room temperature environment. The latter cause is particularly a problem in patients anesthetized for over two hours when there are large incisions exposing the body""s interior to room temperature. Routine measures to reduce heat loss during operation include covering the skin, warming intravenous fluid and transfused blood, and increasing ambient temperature. In most operations, with the exception of those on the brain, prevention of hypothermia is a mainstay of anesthetic management because hypothermia during surgery can adversely affect the outcome. See xe2x80x9cColorectal Surgery Comes in From the Coldxe2x80x9d, The New England Journal of Medicine, Vol. 334, No. 19, Mortensen et al., May 19, 1996.
As discussed above, hypothermia may be encountered as a result of an accident or may be inadvertently acquired during major surgery. In an odd twist, hypothermia may be induced by a physician in the treatment of various conditions to protect the brain or heart. For example, U.S. Pat. No. 5,486,204, issued Jan. 23, 1996 to Clifton discloses a method of treating a non-penetrating head wound with hypothermia. Such a treatment protocol includes specific defined times, temperatures, rates of change of temperature and the timing of the introduction of medications, and controlled rewarming. Additionally, hypothermia is frequently induced during surgery for intra-cranial aneurysms.
The McEwan article notes that treatment of cold injuries has long been controversial. It is also clear that it is not enough merely to reheat a victim suffering from hypothermia, but that controlled heating must be applied. For example, Baron Larrey, Napoleon""s Chief Surgeon observed that those soldiers, suffering from hypothermia, who were placed closest to the campfire during Napoleon""s retreat from Russia died. These soldiers probably rewarmed too rapidly. It is generally accepted that treating hypothermia requires an emphasis on preventing further heat loss, rewarming as soon as it is safely possible at a successful rate (slowly) and rewarming the body core before the extremities in an attempt to avoid inducing lethal side effects during rewarming. This treatment goal is important since hypothermia itself may not be fatal above 25xc2x0 C. core temperature. Fatalities at 25xc2x0 C. or greater normally occur during rewarming.
The McEwan article notices that hypothermia causes several reactions within the body as the body tries to protect itself and retain its heat. One of the most important body reactions is vaso constriction, which halts blood flow to the extremities in order to conserve heat in the critical core area of the body. Shivering also generates peripheral vaso constriction, which minimizes the severity of vascular collapse during rewarming. Induction of vasodilation in hypothermia patients may precipitate rewarming shock and metabolic acidosis. This may occur where the periphery (legs and arms) are warmed before the core (heart and lungs) are warmed. Furthermore, the rapid shunting of cold blood from the extremities to the core, as a result of vasodilation, may cause the core temperature to drop. Prevention of vasodilation is a reason why it is imperative that the hypothermia victim""s extremities not be rewarmed before the core. If vasodilation occurs, cold blood returning to the heart may be enough to put the patient into ventricular fibrillation. Again see, the McEwan article.
The McEwan article notes treatment for the different levels of hypothermia. According to McEwan, treatment for mild hypothermia includes keeping the head and neck covered. Stiff and Sixta note that treatment for mild hypothermia generally includes the application of hot packs, water bottles, or warm campfire rocks wrapped in towels to the groin, head, neck and sides of the chest. McEwan""s treatment for moderate hypothermia includes keeping the head and neck covered, with mild heat applied to the head, neck, chest, armpits and groin of the hypothermia patient. For severe hypothermia, McEwan notes that treatment includes application of heat by skin to skin contact in the areas of the chest and neck with exhaled warm air or steam introduced near the patient""s nose and mouth. Stiff and Sixta note that treatment for severe hypothermia will include application of hot packs to the neck, armpits, sides of chest and groin of the hypothermia victim, with the head kept covered.
Air warmed and cooled devices to maintain normothermia during surgery are available and in wide use. However, as many as 10% of patients are hypothermic during surgery despite use of these devices. They do not contact an adequate amount of body surface to either maintain normothermia during surgery for parts of the body other than the brain, or to safely induce hypothermia during brain surgery or after a head injury. The current lack of devices to effectively control a patient""s temperature may result in poor clinical outcomes.
Prior to discussion of the details of the present invention, reference will first be made to a commonly used prior art blanket. Referring first to FIG. 1, there is shown an illustration of a patient 10 shown positioned on a prior art blanket B. The configuration of a prior art blanket shown generally in FIG. 1 is currently the only configuration commercially available to provide whole body surface cooling. A heat transfer fluid is circulated into and out of blanket B utilizing tubing 11 and 12 respectively. Notice how blanket B generally makes contact with only a limited portion of the skin surface of patient 10, generally the back or front body portion upon which patient 10 is resting. In the supine position, prior art blanket B does not contact the contour of the body. When blanket B contacts the posterior surface of patient 10, it only contacts the scapulae, the buttocks, and the posterior surface of the lower legs. If anterior, prior art blanket B contacts the area of the pectoralis muscles or breasts, the anterior aspect of the abdomen, and the anterior aspect of the upper leg and knee. In addition, in the operating room where a patient is on his side, prior art blanket B would only contact the side of the patient. Furthermore, due to its rectangular shape, prior art blanket B cannot wrap the legs or the trunk, thus leaving the majority of the body surface uncontacted by the blanket. In any of the above situations, the heat transfer area could be improved.
In 1992, one of the inventors utilized a modified non-commercial embodiment of a RotoRest bed (Kinetic Concepts, Inc.) in an hypothermia study. This bed had been equipped with cooling panels for wrapping the abdomen and chest. This bed does not have the capability of warming and cooling different body surfaces at the same time, the cooling apparatus cannot be used independently of the bed, and the bed cannot be used in the operating room or post operative room because of limitations imposed on patient care by the RotoRest bed.
Applicant is unaware of any prior art that discloses or suggests an apparatus for selective rewarming of a hypothermia patient to rewarm various body parts at different rates and at different temperatures to minimize the occurrence of vasodilation. Additionally, such references fail to disclose a suit which wraps the torso and legs leaving the arms, buttocks, perineum and head exposed.
For example, in the situation of a patient suffering from hypothermia or in whom hypothermia has been deliberately induced, exposure of the arms is necessary as they are the primary site for insertion of necessary intravenous lines. Exposure of the head is necessary to maintain control of the airway. The ability to gain ready access to the chest, back and abdomen (the core) is necessary should cardiopulmonary resuscitation be needed, to auscultate heart and breath sounds, to auscultate abdominal sounds or to provide exposure for surgeries of the chest, back or abdomen. Exposure of the legs is necessary for hygiene or for surgery of the legs. The perineum is always exposed in order to provide access to the urinary tract and also because of the significant hygiene issues associated with these sites where body wastes are eliminated. Firm contact of the blanket to the torso and legs, however, is necessary to control temperatures whether inducing hypothermia, maintaining hypothermia or rewarming. In a medical setting, however, ready access to the torso and legs and exposure of head, arms and perineum is required. None of the devices of the prior art meets these needs.
Thus, there is still a need in the art for apparatus for selective heating and cooling of various body parts of a human suffering from hypothermia so that various body parts can be heated and cooled at different rates and at different temperatures.
There is still another need in the art for an apparatus for heating and cooling of a patient in which the maximal body surface is in contact with the cooling/heating surface, which will also provide for easy access to the patient""s body for either surgery or routine patient care, while the patient is being heated and/or cooled.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawing and claims.
The present invention relates to heat transfer blankets which wrap the torso and/or legs leaving the arms, buttocks, perineum, knee, and/or head exposed and allow for the selective heating or cooling of various body parts at the same or different rates.